88
Aging and Sex, DNA Repair in
of recombinational repair in bacterial
transformation as well as in other sexual
processes. The infrequent beneFcial new
traits generated by recombination presum-
ably promote evolutionary success, just as
infrequent beneFcial mutations do.
5.2
Sexual Communication in Fungi, Primarily
for DNA Repair, but Also for Limited
Complementation
1.
Sexual communication in fungi
.Among
eukaryotic microorganisms, pheromones
promote
sexual
interaction
in
many
species. These simple eukaryotes use a va-
riety of molecules, such as steroids, other
lipids, peptides, and derivatives of organic
acids, as well as large molecules such
as glycoproteins as sex pheromones. Al-
though these sex pheromones are usually
transmitted through an aqueous medium,
they may also be transmitted through the
air as in the fungus
Mucos mucedo
.
2.
Sexual communication in yeast
.Th
e
sexual cycle and communication have been
especially well described in the yeasts
Saccharomyces cerevisiae
and
Schizosaccha-
romyces pombe
. Mating in these yeasts oc-
curs between two cells of opposite mating
type and is facilitated by the reciprocal ac-
tion of pheromones. Cells of each mating
type release pheromones that induce mat-
ing through promotion of speciFc mod-
iFcations in cells of the opposite mating
type. The two cells then conjugate, forming
a diploid zygote that can undergo meio-
sis. This leads to sporulation and release
of ascospores that can germinate to form
haploid vegetative cells, thus completing
the life cycle. The pheromones produced
by these yeasts are short peptides.
The results of several studies bear on
the adaptive advantage of the meiotic
events promoted by these sex pheromones.
Recombinational repair of DNA double-
strand breaks occurs during meiosis in
S. cerevisiae
and
S. pombe
.The
rad51
and
dmc1
genes of
S. cerevisiae
are homologs
of the
recA
gene of
E. coli
. A homolog of
E. coli
RecA also occurs in
S. pombe
.
It was found that treatment of
S. pombe
with
H
2
O
2
,aD
N
A
-
d
am
a
g
i
n
ga
g
e
n
t
,
caused increased mating. This stimulation
of mating can be interpreted as an adap-
tation to promote recombinational repair
of the introduced DNA damages. Thus, in
S. cerevisiae
and
S. pombe
,productionofsex
pheromones appears to be an adaptation
for promoting recombinational repair of
DNA, ‘‘Level 1’’ of sexual communication,
as in bacteria.
In
S. pombe
, the mating pheromones
M-factor (produced by the ‘‘Minus’’ mat-
ing type) and P-factor (produced by the
‘‘Plus’’ mating type) ensure sexual inter-
action of cells of opposite mating type.
However, during vegetative growth, a cell
of one mating type switches to the other
mating type in about every two genera-
tions. Similarly, in
S. cerevisiae
,h
ap
lo
id
descendants from spores of one mating
type can change their mating type fre-
quently so that after several cell divisions a
diploid population develops from a single
haploid spore. These observations indicate
that inbreeding is avoided to some extent
in
S. cerevisiae
and
S. pombe
,sincemating
with an individual of the opposite mating
type is required, but that it is not strongly
avoided since a cell of one mating type
can give rise to a cell of the opposite mat-
ing type frequently, and then sibling cells
can mate.
In
S. cerevisiae
and
S. pombe
,thedip
loid
stage of the sexual cycle, formed by mating,
probably expresses functions necessary
for meiosis and sporulation. Recessive
mutations in the genes encoding these
functions may explain the observation that
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